Diabetic retinopathy is a disease caused by progressively impaired blood flow to the retina of the eye. This impaired blood flow eventually leads to oxygen deprivation, or ischemia of the retina. Over time, the ischemia worsens and the retina begins to secrete hormones to produce new blood vessels. These blood vessels are very fragile and grow in inappropriate parts of the eye. They can rupture leading to blindness. Other conditions caused by worsening ischemia in the diabetic eye include macular edema, where the central part of the eye responsible for good vision develops a fluid bubble leading to poor central vision.
The progress of the disease can today be detected by an ophthalmologist, and when the disease is severe enough, it can be treated by burning the retina repeatedly with a laser (panretinal photocoagulation) which stops the secretion of angiogenic hormones (or inhibits their action). There are also drug therapy interventions available and under development that may delay the onset of inappropriate blood vessel development.
Accordingly, early detection of diabetic retinopathy would allow intervention at an earlier stage of the disease allowing better quality of life for diabetics whose vision could be preserved for a longer period of time. The American Academy of Ophthalmology recommends that diabetics that have had the condition for more than ten years get an eye examination annually. Despite the availability of diagnosis and treatment, diabetic retinopathy is the leading cause of blindness in working-age Americans, and is one of the leading causes of blindness worldwide.
At the present, ophthalmologists rely on two primary diagnostic tests for assessment of diabetic retinopathy.                1. Fundus photography is the practice of taking careful photographs of the back of the eye and grading them for the presence of certain characteristics. The photographs are typically taken by a highly trained technician using a specialized camera called a fundus camera. The grading of the photographs is performed either by an ophthalmologist or by specially trained “graders”.        2. Fluorescein angiography involves injecting a fluorescent dye into the patient's vein and photographing the time course of the dye passing through the eye using a specialized camera system. This technique allows the assessment of blood flow across the surface of the eye, and allows an assessment of leakage from the blood vessels. Fluorescein angiography is performed by ophthalmologists or certified technicians and the interpretation of the photographs is performed either by an ophthalmologist or by specially trained “graders”.        
Diabetic retinopathy is not the only disease to cause damage to the eye through the mechanism of retinal ischemia. Less prevalent diseases, including central retinal vein occlusion (CRVO), central retinal artery occlusion (CRAO) and sickle cell anemia may also induce retinal ischemia leading to the growth of inappropriate new blood vessels in the eye.
It has been known for some time that features of the clinical electroretinogram (ERG) are strongly correlated with retinal ischemia and with the extent and severity of diabetic retinopathy. There are many reports in the academic literature describing the relationship between features of the ERG and severity of several types of retinal ischemia. Some of the articles include the following: Bresnick, G, Korth, K, Groo, A. and Palta, M. Archives of Ophthalmology 102: 1307-11 (1984) Electroretinographic oscillatory potentials predict progression of diabetic retinopathy; Bresnick, G. and Palta, M, Archives of Ophthalmology 105:60-664 (1987) Temporal aspects of the electroretinogram in diabetic retinopathy, and Bresnick, G and Palta, M, Archives of Ophthalmology 105: 929-33 (1987) Oscillatory potential amplitudes, Relation to severity of diabetic retinopathy. 
Normally the ERG is recorded using a large instrument in a darkened room with electrodes placed directly onto the eye. Dilating drops are used to enlarge the pupil and anesthetic drops are used to numb the eye before placing the electrodes onto the eye. The waveforms are collected by a skilled technician and are usually interpreted by an ophthalmologist or PhD expert in visual electrophysiology. The aforedescribed disadvantages of the ERG have prevented its widespread use in assessing retinal ischemia.
As can therefore be seen, the present systems and methods of detecting retinal ischemia are by fundus photography, fluorescein angiography or by using a conventional ERG system. All of such methods require expensive equipment and facilities as well as highly trained personnel to operate and interpret the results.
Accordingly, it would, therefore, be desirable to have an inexpensive, hand held device that would be easy to operate, require little or no training to operate and interpret the results. It would be further advantageous to have such a device that could be used by general practice physicians to assess the health of diabetic eyes and thus significantly improve the number of diabetic eyes that are screened for retinopathy.